South Korean and U.S. troops launched computerized military drills Monday despite North Korea warning it would retaliate with a “merciless counterblow” for the exercises Pyongyang considers rehearsal for invasion.

The 11-day drills, dubbed Ulchi Freedom Guardian, are annual war games that involve about 56,000 South Korean soldiers and 30,000 U.S. troops in South Korea and abroad, South Korea’s Defense Ministry and the U.S. command in Seoul said Monday.

No field training is involved in the war games, in which alliance soldiers, mostly senior officers, sit at computers to practice how they engage in battles and hone their decision-making capabilities. The exercises are eventually designed to improve the allies’ joint capability to defend the South and respond to any potential provocations, the U.S. military said in a statement last month.

Washington and Seoul engaged in joint naval drills last month off South Korea’s east coast they called a show of unity after a South Korean warship was sunk in March. The allies blame a North Korea torpedo attack but Pyongyang denies involvement.